This invention relates to a process for the continuous polymerization of a monovinyl aromatic monomer, such as styrene, in an inert solvent, and to an apparatus for such continuous polymerization. Specifically, a process and apparatus are provided which produce a substantially monodisperse polystyrene, i.e., production of a polystyrene which has a low ratio of Mw/Mn (weight-average molecular weight to number-average molecular weight).
It is known to anionically initiate batch polymerization of monovinyl aromatic monomers. Such batch processes, however, suffer the disadvantge of inefficient equipment utilization and batch-to-batch variations in product, for example by inadequate temperature control. A continuous process, on the other hand, eliminates such problems. In a continuous process, however, the apparatus and process steps must be designed for a narrow distribution of residence times. A broad residence time distribution would result in a polymer of broad molecular weight distribution, thus negating one of the principal advantages of anionic polymerization, namely the production of nearly monodisperse polymers.
One method for virtually eliminating residence time distribution is by use of a so-called plug flow reactor. However, for this system, the required length of such a reactor makes it impractical. A second approach is the use of many continuously stirred tank reactors in series. For the required narrow residence time distribution, the number of discrete reactors is far too large to be practical.
Examples of prior continuous polymerization include those described in U.S. Pat. Nos. 3,324,093, 3,262,922, 3,801,555 and 4,311,803. In U.S. Pat. No. 3,324,093, a tilted loop reactor is used for mass polymerization of propylene in the presence of liquid propylene as a diluent, with the effluent from the loop reactor passed to a contact tank, in which tank acetylacetone is admixed with the effluent. The tank has various zones with baffles separating the zones and stirring means in each zone to provide good contact between the acetylacetone and the effluent within the tank. In U.S. Pat. No. 3,262,922, a loop type reactor is used for the mass polymerization of an olefin, with the effluent passed to a treating zone where, as above, acetylacetone is mixed with the effluent. In U.S. Pat. No. 3,801,555, the mass polymerization of conjugated dienes is effected in a reaction vessel which may optionally contain a baffle with the monomer partially polymerized and then passed to a second vessel for further polymerization. In U.S. Pat. No. 4,311,803, a solution polymerization process uses a stirred reaction zone connected in series to a contiguous elongated tubular reaction zone.
In the present invention, a monovinyl aromatic monomer is polymerized in a single vertical reactor which is compartmented into zones and which prevents backmixing between zones. A sufficient number of compartments and elimination of such backmixing reduces the residence time distribution and, thus, the molecular weight distribution of the polymer produced.